On the gray dawn of Monday 5th June, 1944, rain lashed into the German bunkers and large waves pounded the beaches of France. This was the morning originally chosen for the Allied invasion of Europe, but the Allies postponed the invasion by 24 hours. This decision saved the Allied forces from certain destruction in the English Channel.

Six forecasters working in three different teams were responsible for the D-Day forecasts. The American team used an analogue method that compared the current weather with past conditions. Their forecast was overly optimistic and the British Admiralty and the British Meteorological Office urged delay. They were aided by the brilliant Norwegian theoretician Sverre Petterssen who used high altitude observations in his forecasts.

Met Office chart from 5th June 1944

In the early hours of June 5th, under stormy skies of England, the forecasters advised General Eisenhower that a very short break in the weather a day later would allow the invasion to go ahead. On Tuesday, June 6th, under barely tolerable conditions, the largest amphibious landing force ever put together landed on the beaches of Normandy.

Ironically the German meteorologists were aware of new storms moving in from the North Atlantic, but they had decided that the weather would be too bad to permit an invasion attempt. The Germans were caught completely off guard. Their high command had relaxed and many officers were on leave; their airplanes were grounded; their naval vessels absent.

This marked the beginning of the end of the war in Europe and it depended on what were arguably the three most critical forecasts in history -- two successful ones by the Allies and one failure by the Germans